In a typical modern locomotive, a diesel engine is used to provide mechanical energy to a electric generator. The generator converts this mechanical energy into electrical power which is used to operate a plurality of direct current (dc) traction motors, each driving a separate drive axle having a pair of drive wheels connected thereto.
Wheel slip usually occurs during acceleration, and can take two forms. The first type of wheel slip is referred to as differential wheel slip which occurs when at least one set of drive wheels maintains tractive contact with the rail while at least one set of the remaining drive wheels slip. A second type of wheel slip is synchronous slip which occurs when none of the drive wheels maintains tractive contact with the rail and all of the drive wheels slip more or less simultaneously.
Wheel slip has long been a problem in locomotives and many systems have been developed which either reduce or completely eliminate wheel slip. A common scheme is to compare speed signals from driven and idler wheels or speed signals from each of several driven wheels or highest and lowest speed signals from traction motors. A slip condition is presumed to exist if the compared speed signals differ by more than a preselected magnitude. In the above mentioned systems, sensors, such as speed transducers, are used to produce the speed signals and such sensors add extra costs to wheel slip control systems. Furthermore, such systems are usually directed towards controlling only differential wheel slip and, as mentioned above, synchronous wheel slip also degrades locomotive performance.
A system for limiting both synchronous and differential wheel slip is disclosed in U.S. Pat. No. 4,463,289 which issued on Jul. 31, 1984 to Young. Beginning on line 35 of column 4, Young discusses a rate circuit for controlling synchronous wheel slip. The rate circuit provides a signal representing the rate of acceleration of the wheel with the greatest velocity. If the measured wheel acceleration is greater than a predetermined value, a rate signal is applied to the generator exciter which reduces exciter output and wheel speed. However, this scheme still requires sensors to detect the speed of each drive wheel.
The present invention is directed towards addressing the above mentioned problems by controlling synchronous wheel slip in a locomotive without requiring speed sensors. Other aspects, objects and advantages can be obtained from a study of the drawings, the disclosure, and the appended claims.